The invention relates to a catalyst for the steam reforming of hydrocarbons which has improved thermal shock resistance and contains nickel on an alumina/calcium aluminate support.
Steam reforming of hydrocarbons is the hydrogen production process which is at present the most important on an industrial scale. Steam reforming of hydrocarbons is an endothermic reaction, and the heat required for the process must be supplied to the reaction system from the outside. An industrially important variant of this process is autothermic steam reforming, wherein the heat balance of the reaction is maintained by partial oxidation of the hydrocarbons, in particular methane.
Steam reforming poses very stringent requirements on the mechanical properties of the catalysts, especially of the supports. The catalysts must have a high mechanical and thermal stability and thermal shock resistance at temperatures above 800.degree. C., especially above 1000.degree. C., and pressures of around 30 bar in an atmosphere consisting essentially of steam, hydrogen and carbon dioxide. On the other hand, in order to achieve a high activity, the catalysts must have a support matrix having an open pore structure. Hitherto, it has been possible to meet this requirement only at the expense of reducing the mechanical and thermal stability of the catalysts.
Mechanical strength is in general understood as lateral crush strength, which is measured in the known manner on catalysts in the form of pellets or rings.
The ability of a catalyst to withstand, without fracture, sharp temperature changes and the resulting high thermal stresses, that is to say the mechanical stresses caused by thermal treatment, is called thermal shock resistance. If the thermal stresses at any point in the molding exceed critical value (the "crushing strength"), cracks form at this point and can lead, under some circumstances, to complete fracture of the molding.
Attempts have already been made to improve the thermal stability of the supports (mainly based on alumina) of nickel-containing catalysts for the steam reforming of hydrocarbons by the use of certain additives.
Thus, for example, the alumina supports described in European Patent Application 130,835 contain oxides of certain rear earths.
West German Patent No. 2,431,983 describes a nickel catalyst, containing a refractory calcium aluminate, for the steam reforming of hydrocarbons, wherein the support contains 10 to 60 percent by weight of calcium oxide, 0 to 30 percent by weight of beryllium oxide, magnesium oxide and/or strontium oxide, 30 to 90 percent by weight of alumina and less than 0.2 percent by weight of silica. The increase in strength is ascribed mainly to the use of calcium aluminate of low silica content.
Finally, the literature reference Khim. Tekhnol. (USSR) 5, (1979) has disclosed that the mechanical and thermal properties of catalysts for the steam reforming of methane can be improved by adding minor quantities of other oxides to the alumina used as the support. The addition of CaO, Sc.sub.2 O.sub.3 or TiO.sub.2 has here proven to be particularly favorable. This literature reference does not, however, disclose calcium aluminate in supports based on alumina in combination with an addition of small quantities of TiO.sub.2.